Tankless Size Chart

Flow rate and temperature rise of small tankless water heaters for ADUs, cabins and off-grid buildings

Section 1

Tankless water heater sizer by fixture flow and temperature rise

The sizer adds the flow of the fixtures that run hot at the same time. It subtracts the inlet water temperature from the outlet temperature to get the rise. It then compares each model's rated flow at that rise.

1.1 Fixtures running at the same time

Preset flows come from section 4. Each flow can be changed to match the fixture's rating.

1.2 Inlet water

The preset is the colder end of the range on Rinnai's ground water map. Winter water can be colder than the map.

1.3 Outlet temperature

The result appears here once the page script loads.

1.4 How the sizer works

  1. Required flow is the sum of the fixture flows entered.
  2. Rise is the outlet temperature minus the inlet temperature.
  3. Each model's capacity is the smallest GPM-times-rise product among the manufacturer's rated points (section 2). Capacity divided by the rise gives the flow the model delivers. A listed maximum flow caps the result.
  4. For models with no listed maximum flow, a rise below the lowest rated rise is treated as that lowest rise. This keeps the result inside the manufacturer's table.
  5. Models with a rating that is not confirmed in section 9, and portables with no rated rise, are listed but not counted as a match.

The electrical service, breaker sizes and gas venting shown with the results are the manufacturer's published requirements. They are not installation instructions. A licensed electrician or gas installer decides what a building can take.